U.S. Supreme Court nominee judge Neil Gorsuch testifies during a third day of his Senate Judiciary Committee confirmation hearing on Capitol Hill in Washington, U.S., March 22, 2017. REUTERS/Jonathan Ernst/File Photo

The U.S. Senate fight over President Donald Trump’s Supreme Court nominee, Neil Gorsuch, begins moving on Tuesday to its decisive phase as Democrats seek to block a final vote and Republicans vow to ensure the conservative judge’s confirmation.

Democrats amassed enough support on Monday to block a straightforward Senate confirmation vote, but Republicans threatened to change long-standing Senate rules to force a final vote by the end of the week, a move known as the “nuclear option.”

Senate confirmation of Gorsuch, 49, an appeals court judge would restore the nine-seat high court’s conservative majority, fulfilling one of Trump’s top campaign promises. The vacancy was created by the February 2016 death of conservative Justice Antonin Scalia.

The Senate’s Republican leaders insist Gorsuch will be confirmed on the Senate floor on Friday regardless of what the Democrats do. Republicans hold a 52-48 Senate majority.

The Senate Judiciary Committee voted 11-9 on Monday along party lines to send Gorsuch’s nomination to the full Senate.

The extreme partisan divisions in the chamber illustrate the difficulties Trump faces in pushing through his agenda even though his fellow Republicans control the White House and Congress.

The procedural hurdle backed by Democrats, called a filibuster, requires a super-majority of 60 votes in the 100-seat Senate to allow a confirmation vote.

The process leading to the showdown is likely to begin on Tuesday when Republican Senate Majority Leader Mitch McConnell sets the clock ticking on the Senate floor debate on the nomination. The Senate would then vote on Thursday on whether to limit the debate.

Under Senate rules, if at least 41 Democrats vote against ending the debate, no final vote on the nomination is possible. In the face of the filibuster, McConnell would be expected to force through the rule change, leading to a final vote on Friday evening to confirm Gorsuch by a simple majority.

NOT GIVING GROUND

McConnell said on Monday it was not too late for Democrats to back off, calling on them to “make the right choice” and not seek to block Gorsuch. He said there was no principled reason to oppose a judge as well qualified and widely respected as Gorsuch.

Democratic Senate leader Chuck Schumer gave no signs a detente was possible, saying that instead of changing the Senate rules to navigate around Democrats’ opposition, Republicans should pick a more mainstream nominee for the lifetime appointment.

“They can choose to go nuclear or they can sit down with Democrats to preserve the grand traditions of this body,” Schumer said.

Democrats have accused Gorsuch of being insufficiently independent of Trump, evading questions on key Supreme Court rulings of the past including on abortion and political spending, and favoring corporate interests over ordinary Americans.

Democratic senators are still angered by the refusal of Senate Republicans last year even to consider former Democratic President Barack Obama’s nomination of appellate judge Merrick Garland to fill the same high court vacancy that Trump has selected Gorsuch to fill.